Apparatus for manufacture of void-free and warp-free slab stock

ABSTRACT

Void-free slabs or billets of thermoplastic polymer are produced by cooling polymer melt in a mold under pressure while keeping part of the melt in contact with a heat-insulating deformable surface of the mold cavity. Apparatus of this invention comprises a mold having two parallel rigid planar surfaces, a deformable heat-insulating ring placed between said planar surfaces, stiffening or backing means to maintain the shape of the outer wall of the ring, and pressure means for controllable changing the distance between said surfaces.

United States atent 1 1 1111 3,733,159 Coffman 1 May 15, 1973 [54]APPARATUS FOR MANUFACTURE OF [56] References Cited VOID-FREE ANDWARP-FREE SLAB UNITED STATES PATENTS STOCK 2,172,243 9/1939 Goodnow 6ta1........425/DIG. 44 UX Invent Paul Cmma", Cherry H111, 3,079,6423/1963 Needham et al ..264/320 x [73] Assigneez o Company, New York3,478,135 11/1969 Randall ..425/812 UX 2,714,226 8/1955 Axelrod..425/D1G. 44 UX Flledl g- 5, 1971 2,965,946 12/1960 Sweet et a1.......425/D1G. 3s ux 3,235,643 2/1966 Hofer, ..425/352 X [21] Ap l flfios1159,4 22

Primary Examiner-Robert L. Spicer, Jr. Related Application Data IAttorney- Martin S. Baer et a1. [63] Continuation-impart of Ser. No.857,176, Sept. 11,

1969, Pat. No. 3,608,508, which is a continuation-in- [57] ABSTRACT p r,1 April 1967, abandoned, Void-free slabs or billets of thermoplasticpolymer are which is a continuation-in-part of Ser. No. 524,743,produced by cooling polymer melt in a mold under 1966 abandoned pressurewhile keeping part of the melt in contact with [52] U S 425/186 425/352425/406 a heat-insulating deformable surface of the mold cavi- 264/313264/320 ty. Apparatus of this invention comprises a mold hav- 264/325ing two parallel rigid planar surfaces, a deformable 51 Int. Cl..B29c'3/00 heat-insulating ring placed between Said Planar [58] Fieldof Search ..425/394, 406, 193, f stiffening or backing means to maintainthe 425/195, 411, 352, 354, 812, DIG. 35, DIG. 44, 186; 264/320, 313,325

shape of the outer wall of the ring, and pressure means for controllablechanging the distance between said surfaces.

4 Claims, 12 Drawing Figures PATENTEDHAYIEIQYS 3,733,159

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PAUL M. COFFMAN HIS ATTORNEY PATENTEDH 3,733,159

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PAUL M. COFFMAN HIS ATTORNEY APPARATUS FOR MANUFACTURE OF VOID-FRIEIEAND WARP-FREE SLAB STOCK This application is a continuation-in-part ofSer. No. 857,176, now U.S. Pat. No. 3,608,508, which is acontinuation-in-part of Ser. No. 633,673 which is a continuation-in-partof Ser. No. 524,743, both now abancloned.

This invention relates to apparatus for producing thermoplastic articlesand particularly for producing slab-stock or billets of substantialweight and thickness, free of internal voids and other defects, adaptedto conversion to useful articles by forming without remelting. Theinvention is particularly adapted for use in the formation of largerelatively thick slabs of solid crystalline polypropylene and highdensity polyethylene. Methods of utilizing the apparatus of thisinvention are described and claimed in parent U.S. Pat. No. 3,608,508.

It has been found that thermoplastics such as polyethylene andpolypropylene, including those of very high molecular weight, can beconverted to shaped articles by rapid forming operations applied tobillets of polymer at a temperature below its melt temperature. Toprovide fault-free articles the billet must be of suitable shape,usually smooth walled, and free of surface defects as well as ofinternal defects such as internal voids, stresses or flow lines.

The main object of this invention is to provide apparatus suitable forproducing in an economical manner slabs and billets of thermoplasticpolymer, particularly of polypropylene or high density polyethylene,which are free of internal voids, surface sinks, warp and other defectsand can be converted into useful articles by shaping methods in whichthe polymer remains unmelted.

' Other objects of this invention will appear from the followingdescription, in which reference is made to the drawing wherein:

FIGS. 1, 2 and 3 illustrate one form of apparatus of this invention,FIG. 1 being a diagrammatic view in vertical section of apparatus inwhich a molten plastic mass is in position for molding, FIG. 2 being inview of the apparatus of FIG. 1 while the plastic mass is under moldingpressure; and FIG. 3 being a view of the apparatus and molded article oncompletion of the molding cycle;

FIGS. 4 and 5 illustrate an alternative apparatus of this invention,FIG. 4 being a diagrammatic view in vertical section of apparatus inwhich a molten plastic mass is in position for molding and FIG. 5 beinga view of the apparatus of FIG. 4 while the plastic mass is undermolding pressure;

FIG. 6 is an isometric view of an article produced in the apparatus ofthis invention;

FIG. 7 is an isometric view of a defective article, produced withoututilizing this invention;

FIGS. 8, 9, l0 and 11 are isometric views, some partly cut away, ofshapes of deformable die members which can be utilized in the apparatusof this invention; and

FIG. 12 is a schematic view of an apparatus assembly illustrating theprocess of forming articles when utilizing this invention.

One of the characteristics of polypropylene and high densitypolyethylene is the large shrinkage in cooling from a liquid to a solidmaterial. For example, polypropylene shrinks about 15 percentin volumeand high density polyethylene about percent in cooling from 400F to roomtemperature. In conventional methods of cooling a melt of such polymerin a mold, the surface of the polymer mass hardens while the interiorpart remains molten. As the interior also finally solidifies, surfacesinks, internal voids and internal stresses are developed, especially inthick sections. Solid articles containing such internal voids andstresses are generally not suitable for conversion to useful articles byforming or finishing methods in which the polymer remains unmelted.Conventional molding is therefore not adequate for forming thickarticles such as slabs or billets, free of internal voids and otherdefects.

A method for molding large void-free polyolefin blocks is disclosed byNeedham et al. in U.S. Pat. 3,079,642. The method comprises placingmolten polymer in a block mold, applying pressure while the polymer iscooled until a hardened skin is formed, removing the partially cooledblock from the mold and permitting cooling to be completed whilepressure is applied to two faces of the block, the remainder of theblock remaining unconfined. As pointed out in the patent, the skin ofthe unconfined portion of the block will tend to rupture unless care istaken not to apply excessive pressure. Insufficient pressure, or theother hand, results in a block which is not void-free. A necessaryresult of the method of Needham et al. is that a block is formed whichhas convex sides, for example, a 1-inch block has sides that extend atthe center one-fourth inch beyond the edges of the upper and lowersurface. Substantial trimming, with consequent waste, is necessary toproduce blocks having straight sides.

The apparatus of the present invention makes possible the production oflarge, thick, molded thermoplastic articles, such as slabs or billets,which are substantially free of internal voids and other defects withoutthe disadvantages encountered in the method of Needham et al. Thesearticles may be useful as such, but are especially useful for furtherconversion into other articles of substantial thickness by conversionmethods not requiring remelting of the polymer. The apparatus of thisinvention is especially adapted for production of articles which havestraight side walls.

The apparatus of this invention is quite inexpensive compared toapparatus required for producing thermoplastic shapes by methods such asinjection molding, blow molding or extrusion. Slabs and billets producedby this invention are relatively inexpensive and can be converted tofinished articles of more complex shape at a high rate in an economicalover-all process.

Referring now to the drawing, wherein corresponding parts are designedby the same numeral, a preferred embodiment of the apparatus and its usein the method of forming a plastic article are illustrated in FIGS. 1, 2and 3. The principal parts of the apparatus are a rigid lower press orplaten member 10, having a flat surface lll; a rigid upper press orplaten member 12, having a fiat surface 13; a deformable die member 14a;a rigid backup ring 15; and a shim 16 which supports the backup ring.

The upper and lower platen may be individually movable toward and awayfrom each other, or one may be stationary and the other movable. Theirsurfaces are parallel. The surfaces 11 and 13 constitute the upper andlower surfaces of the die in which the polymer billets are formed. Atleast one and preferably both the platen members are adapted to coolpolymer in the mold.

The circumference of the plastic article is defined by die member 14a,which is a deformable body made of material which is essentially rigidwhen not subjected to force, is capable of flow under molding pressure,and is a poor conductor of heat.

The deformable die member is typically made of rubber or of arubber-like material or composition; this includes any elastomericmaterials such as vulcanized rubber and synthetic elastomers, which havesufficient rigidity to retain their shape when not under pressure andwhich are within their elastic limits capable of flow similar to that ofa liquid when under pressure during the molding process. The chemicalcomposition of the elastomer is not important from the point of view ofthis invention, provided it has the desired physical characteristics,including poor heat conductance, is not degraded at the highesttemperatures to which it will be exposed during the molding operation,and is chemically inert to the thermoplastic charge.

The deformable die member or die ring may be of circular, rectangular orany other desired configuration. A circular and a square die ring areillustrated in FIGS. 8 and 9 as rings 14a and 14b, respectively.

An important feature of die ring 14a is that its upper surface isgrooved or notched at intervals which radial slots, grooves or notches20. These serve to permit captured air to escape during the moldingprocess. There should be at least four such slots spaced about 90 apart,but a much larger number may be employed if desired.

Omission of the radial air escape grooves may cause the production ofbillets having deformities in the upper surface due to air retainedbetween the die ring and upper platen. It is, however, within the scopeof this invention to operate with a die ring having an ungrooved uppersurface.

A further important feature of the apparatus of this invention is theuse of means for reinforcing or stiffening the outer wall or perimeterof the die ring, as further described and illustrated.

The wall configuration of the die ring affects the shape of the plasticbillet formed in the process. In the method of Needham et al., forexample, the plastic mass solidifies while unconfined at its sides,resulting in a block having convex sidewalls. A billet having convexsidewalls is also obtained if the deformable die ring used in thisinvention has a straight wall, especially if it is used either without abackup ring or other stiffening means in the center part of its outerwall or with a backup to its full height, e.g., when it is used as linerfor a right cylindrical die.

The die ring is selected so that the volume enclosed by the undeformeddie ring and the rigid die surfaces is about the same as the volume ofpolymer melt needed to produce the desired billet. The height of the diering is generally 115-135 percent of the height of the finished billet.

The grooves in the die ring may be narrow slots, e.g., one-sixteenthinch by one-sixteenth inch in section. Their dimensions are notcritical. They should be relatively shallow, e.g., percent or less ofthe height of the die ring, and should in any event terminate above thelevel of the finished billet. The grooves may be omitted, but at therisk of producing billets having irregular upper surfaces.

Preferred embodiments of the die ring are illustrated in FIGS. 1-3 and 8(a partly cut-away isometric view).

Here the die ring has an inner wall of concave curvature.

When used with a rigid backup ring whose height is about one half of theheight of the finished billet, as illustrated, the latter will havecompletely or substantially straight sidewalls. The backup ring ispreferably from one-fourth to three-fourths of the height of the billetbeing formed and should be vertically centered, being supported by asuitable shim, as illustrated.

In a typical arrangement, to produce a billet having a height of lOOunits, dimension 0 in FIG. 1 would be 50 units and b would be units,with the ring 16 being centered so that the height of shim 16 is about40 units.

In an alternative embodiment of this invention, the deformable body, isan elastomeric ring which contains reinforcing or stiffening means at ornear its outer wall.

Apparatus including a reinforced die ring is illustrated in FIGS. 4 and5, and suitable reinforced die rings in FIGS. 10 and 11. The apparatusconsists of two opposed, flat-surfaced rams l0 and 12 having rigid,flat-surfaced rams 10 and 12 having rigid, flat surfaces 11 and 13 and adie ring 140 acting as deformable die. As shown, the outer wall of diering 140 is reinforced by embedded wires 17. Other stiffening means maybe employed, such as band of metal or firm fabric adhesively secured tothe outer wall of the die ring, preferably to the center third thereof,as shown in FIG. 11.

In another modification of the invention, not shown, the die ring may bereplaced by a fluid-filled solid deformable toroid, utilized withreinforcement of its outer perimeter.

Conventional parts of the apparatus which are not required to explainthe present invention are not shown in the drawing. For example, theconventional heating and cooling means which permit controlling thetemperature of the rigid die members are omitted. Also not shown are thepress employed for forcing the die surfaces together during the moldingoperation, and associated equipment such as means for removing thecompleted article.

In the formation of void-free thermoplastic articles by compressionmolding utilizing the apparatus of this invention, FIG. 1 illustratesthe position of the dies after a molten plastic mass M has been placedon platen surface 11. Platen members 10 and 12 are then urged towardeach other. One may be stationary or both may be moved. When the uppersurface 13 contacts the plastic mass it forces the mass to conform tothe shape defined by die surfaces 11 and 13 and deformable die member14a. As the mold cavity closes, upper platen surface 13 first contactsplastic mass M, causing the mass to flatten and flow out toward diemember 14a. When upper platen surface 13 first contacts the uppersurface of the die ring, the plastic mass has not completely filled theavailable space. At that point, air would be trapped in the die if slots20 were not provided. With the slots present, trapped air escapes as thegap between the platens continues to decrease. When the polymer hascompletely filled the available mold space, the pressure is maintainedbut there is no further substantial immediate movement. Cooling is thencarried out by circulating cooling fluid through at least one, andsuitably both platens. The elastomeric material of the liner acts asinsulator and prevents or slows down hardening of the molten plasticmass with which it is in contact, so long as a substantial portion ofthe total plastic mass M is still molten. Accordingly, while the plasticmass cools and shrinks, the portions thereof which contact the cooleddie surfaces harden first. Pressure is maintained while the plastic masscools and shrinks. This mold pressure forces elastomeric insert 14agradually to deform, by liquid-like flow, into the configurationillustrated in FIG. 2. Since the molten portion of the plastic mass isunder continuous molding pressure, it solidifies into a void-free,substantially strain-free solid body B.

The molding operation illustrated in FIGS. 4 and 5 is essentially likethat of FIGS. 1 and 2. FIG. 4 illustrates the apparatus which moltenplastic mass M in place and FIG. 5 after full molding pressure isapplied.

FIG. 6 illustrates a circular billet B formed according to thisinvention; a segment is cut away for easier understanding of its shape.The top and bottom surfaces are flat and smooth, conforming to diesurfaces llla and 12a. The edges are essentially straight as a result ofthe use of concave-walled die ring 14a and backup ring 15.

FIG. 7 illustrates what happens when a plastic mass is compressionmolded in a rigid die mold having a straight, rigid die-wall surface.After the mass has solidified, it is found to contain internal voids l9and to have a concave side wall and concave, more or less lensshaped topand bottom surfaces 18. While the lensshaped surfaces can be machined toplane surfaces, albeit with substantial wastage of material, theinternal voids and associated stresses make such a molded articleuseless for many further forming operations.

Several possible shapes of the elastomeric insulating mold member areillustrated in FIGS. 8, 9, and 11. FIG. 8 illustrates a ring, 14a,having a concave inside wall, as used in apparatus of FIGS. L3. FIG. 9illustrates a square tubular mold member. FIG. 10 illustrates thewire-reinforced ring 140 used in the apparatus of FIGS. 45. FIG. 11illustrates a ring reinforced by an outer band 17a, which may be astrong, nonelastic fabric, a metal, or other suitable reinforcing means,adhesively or otherwise secured to the outer wall of die ring 14d. Whileit is preferred to reinforce about the central third of the outer wall,it is within the scope of the invention to extend the reinforcement andeven use a die ring with a completely reinforced outer wall.

FIG. 12 illustrates a preferred mode of utilizing the apparatus of thisinvention. Thermoplastic polymer to be molded is charged in any desiredform, e.g., as nibs or pellets, through line 21 to machine 22 whereinthe polymer is converted to a molten mass. The machine is typically ascrew extruder. The molten polymer is typically quite viscous, having,for example, a viscosity in the range of 10 to 10 'poise. A stream 23 ofsuch melt is separated into portions 24 of predetermined weight. Forexample, melt may be collected on a sheet or thin plate 25 which has asurface to which the polymer does 'not adhere, e.g., a surface coatedwith a fluorohydrocarbon polymer such as polytetrafluoroethylene. Thesheet or plate rests on a table 26 which may be a weighing table or aportable table on a scale. The surface of table 26, or sheet 25, isheated or heavily insulated to prevent premature cooling of the polymermelt. Sheet 25 or table 26 is removed and replaced by another after aslab of predetermined weight has been collected. Polymer mass 24 isthicker, but smaller in area, than the mold. It is transferred to themolding station where it is placed on the surface 11 of platen 10.Rubber die ring 14a, shim 16 and backup ring 15 are then placed on diesurface 11. The die is then closed and the melt is molded into avoid-free, strain-free article as previously described. The mold isopened, and the article is removed from the surface of platen 10, andtransferred, is necessary, to a trimming station, not shown, where anyirregular edge portion of the billet is trimmed off, leaving it a smoothsided disc capable of further conversion to a useful article by otherforming methods. Other trimming operations may be performed, e. g.,planing the top and bottom surfaces, drilling holes or the like. Theprepared billets are suitable for conversion into finished articles ofmore complex shapes by forming operations in which the polymer undergoesflow while in the solid state.

Preferred embodiments of this invention have been described. Theapparatus of this invention is, however, capable of other modificationswhile remaining within the scope of this invention. An example of asimple modification includes the use of the yieldable die ring as insertin a rigid cylindrical die.

Conditions for utilizing the described apparatus in the molding ofvoid-free billets are detailed in the parent patent hereof, US. Pat. No.3,608,508. The invention leads itself readily to the production ofbillets of one-fourth inch or greater thickness. Billets of one-halfinch to 2 inch thickness are particularly useful. The molding pressurerequired for producing satisfactory void-free articles is generally inthe range from 10 to 400 psi. While higher pressures can be used withoutadverse effects, they are not ordinarily required. With a solid rubberliner the pressure required to mold voidfree articles is generallybetween about 50 and about psi, but lower pressures, down to about 20psi minimum may also give useful results, and higher pressures may beemployed if desired. With an inflated liner, molding pressures of 10 to20 psi are generally preferred and even lower pressures may besufficient.

The melt should be at a temperature above its melting point when placedin the mold. If the mold is heated to about the temperature of the melt,the resulting article can have top and bottom surfaces equal in thequality to the mold surfaces. However, polymer may be placed intorelatively cool mold, e.g., one at 60-200F, to shorten the cycle time.In that case, the surface which contacts the cool mold may haveimperfections but these can be readily trimmed off the molded article.

After the mold is closed and under pressure, at least one, andpreferably both, of the heat-conducting dies are cooled, e.g., bycirculating cooling water through internal channels, the known manner.The article must remain in the mold under pressure until it iscompletely solidified.

The apparatus of this invention can be used as illustrated for formingindividual thick billets for further reshaping. The invention is ofadvantage for the production of large, thick slabs of polymers fromwhich individual billets can then be cut for further reshaping, e.g., informing slabs or billets of at least 2 ft area and especially those ofat least about 10 ft area with thickness from one-half inch to inches orgreater, and weighing up to about 500 pounds.

The cooling time required to solidify an article in the Y mold variesapproximately as the square of the thickness. A time of at least about 5minutes is required to solidity a polypropylene article of one-half inchthickness; articles of 1 inch, 1% inch, and 2 inch thickness require atleast about 20, 45 and 80 minutes cooling time for completesolidification.

The invention is of special advantage in forming voidfree billets fromcrystalline polymers, such as linear or high-density polyethylene andisotactic polypropylene and other stereo-regular thermoplasticpolyolefins. It may be employed to advantage in forming billets fromother thermoplastic polymers, such as polyamides (Nylon), polyacetals,polycarbonates, polyvinylchlorides, and the like, and is of particularadvantage for those polymers which shrink substantially while coolingfrom the melt.

The polymer compositions may contain various types of known additives,such as stabilizers against thermal or actinic degradation; pigments ordies; and fillers of various types, including powdered materials andfibrous materials, e.g., talc, glass fibers, asbestos, etc.

I claim as my invention:

1. Apparatus for producing plastic billets comprising a mold consistingof two parallel rigid planar surfaces and a deformable, heat-insulatingring having flat bottom and top surfaces, arranged so that upon closureof the mold each of said bottom and top surfaces of said ring is incontact with one of said planar surfaces and the mold cavity is definedby portions of said rigid planar surfaces and the interior surface ofsaid ring; reinforcing means for from about the central one fourth toabout the central three fourths of the outer wall of said ring; andpressure means for controllably changing the distance between said moldsurfaces.

2. Apparatus according to claim 1 wherein said ring has at least fourradial grooves in its top surface, spaced about equally from each otherand having a depth no more than 15 percent of the height of the billetto be formed in said mold.

3. Apparatus according to claim 2 wherein said reinforcing meanscomprises a band of substantially nondeformable material around at leastone third of the height of the outer wall of said ring.

4. Apparatus according to claim 2 wherein the inner wall of said ring isof concave curvature.

1. Apparatus for producing plastic billets comprising a mold consistingof two parallel rigid planar surfaces and a deformable, heat-insulatingring having flat bottom and top surfaces, arranged so that upon closureof the mold each of said bottom and top surfaces of said ring is incontact with one of said planar surfaces and the mold cavity is definedby portions of said rigid planar surfaces and the interior surface ofsaid ring; reinforcing means for from about the central one fourth toabout the central three fourths of the outer wall of said ring; andpressure means for controllably changing the distance between said moldsurfaces.
 2. Apparatus according to claim 1 wherein said ring has atleast four radial grooves in its top surface, spaced about equally fromeach other and having a depth no more than 15 percent of the height ofthe billet to be formed in said mold.
 3. Apparatus according to claim 2wherein said reinforcing means comprises a band of substantiallynon-deformable material around at least one third of the height of theouter wall of said ring.
 4. Apparatus according to claim 2 wherein theinner wall of said ring is of concave curvature.